Switched-mode power controllers or converters may be designed in a wide variety of different ways and may use a variety of different control mechanisms to convert an input current into an output current (e.g., convert an input alternating current to an output direct current, or convert an input direct current level to a different output direct current level). In operation, a quasi-resonant switched-mode power controller typically provides a pulse-width modulation (PWM) signal that periodically turns on and off a power switch. The power switch typically supplies current to an inductive device (e.g., an inductor or transformer). The time between two rising edges of the PWM signal corresponds to a switching cycle that consists of an on-time and an off-time. The inductive device magnetizes during the on-time and demagnetizes during the off-time.
At the completion of demagnetization, the inductive and parasitic capacitance components induce a quasi-resonant oscillating signal that includes valleys at times when the voltage across the power switch is zero or minimum value. Some quasi-resonant switched-mode power controllers are designed to minimize switching losses by turning the power switch on at selected valley times during off-time cycles (referred to as “zero-voltage valley switching”). In this process, the quasi-resonant switched-mode power converter typically includes a valley detection circuit that produces a valley signal for each valley detected in the oscillating auxiliary winding signal.
Based on a comparison of the output voltage of the quasi-resonant switched-mode power controller with a reference voltage, a compensation circuit typically generates a compensation signal that can be used to regulate the durations of the main switch on-times and off-times to maintain the output voltage at the target level.